Field of the Invention
The present invention relates to thermogravimetric techniques for determining the composition, phase, structure or other properties of a sample of material.
Thermogravimetric Analysis ("TGA") techniques generally comprise measuring the weight change of a material as a function of temperature, or as a function of time at a controlled temperature. The sample temperature is strictly controlled throughout the analysis. The classic TGA method comprises heating the sample at a constant heating rate, e.g., at 10.degree. C. to 50.degree. C. per minute, while the weight change or the percent of weight change of the sample is recorded versus temperature. Whenever the sample undergoes a chemical or physical transformation that affects the weight of the sample, the change in weight may be interpreted to analyze the composition, structure, or thermal stability of the sample.
There are two general types of TGA instruments: vertical TGA instruments and horizontal TGA instruments. Vertical TGA instruments support the sample/reference holders vertically. Horizontal TGA systems support the sample/reference holders at the end of a horizontal beam. The present invention applies to horizontal TGA instruments.
TGA instruments can utilize either direct or indirect temperature measurement. Direct temperature measurement designs have the temperature sensor in physical contact with either the sample or the sample holder. The sensor is in physical contact with the sample or the sample holder when there exists a low thermal resistance path between the sensor and the sample or the sample holder. Indirect temperature measurement designs have the temperature measuring sensor separated from the sample and the sample holder by the atmosphere which surrounds the sample. Each type of temperature measurement has advantages. The main advantage of indirect temperature measurement lies in the reduced complexity of the balance beam and temperature measuring apparatus. The main advantage of direct temperature measurement lies in the increased accuracy of the temperature measurement because thermal gradients between the sample and the sensor are significantly reduced.
Differential Thermogravimetric Analysis ("DTGA") instruments measure the weight change of a sample of the material with reference to the weight change of a reference material, as a function of the temperature of the sample, or as a function of time at a controlled temperature. DTGA instruments compensate for the effects of heating rate and ambient conditions that could cause changes in the measured weight of the sample and reference. The DTGA technique can increase the sensitivity of the measurement of the change in sample weight by removing large offsets in the value of the sample weight whenever the precision of the measuring apparatus is limited.
Differential Scanning Calorimetry ("DSC") is a thermal analysis technique which measures the temperatures and heat flow associated with transitions in materials as a function of time and temperature. These measurements provide quantitative and qualitative information about the sample transitions that involve endothermic or exothermic processes, or changes in heat capacity.
Differential Thermal Analysis ("DTA"), like DSC, measures the temperatures and heat flow associated with transitions in materials as a function of time and temperature. However, unlike DSC, DTA results are semi-quantitative. DTA is generally carried out at higher temperatures than DSC.
Simultaneous TGA/DTA and simultaneous TGA/DSC, measure weight change and differential temperature changes simultaneously in a sample and a reference, as a function of temperature, or as a function of time at a controlled temperature. Simultaneous measurement of these two physical properties improves productivity of the measurement and simplifies interpretation of the results. The complementary information obtained from the simultaneous measurement allows differentiation between events which have no associated weight change (e.g. melting and crystallization), and those which involve a weight change (e.g. degradation and evaporation).
The simultaneous measurement of weight changes and differential temperature changes on the same sample also assures identical experimental conditions and data sampling for both measurements, thereby eliminating those sources of uncertainty.
Differential temperature measurements generally require the use of a direct temperature measurement design because the differential temperatures being measured are typically not greater than the thermal gradients which occur with indirect temperature measurements.
Conventional horizontal simultaneous TGA/DTA, and simultaneous TGA/DSC instruments are generally constructed with either one or two support beams. The beam(s) extend from the balance pivot point, which is located outside the TGA furnace, to the sample support point, which is located inside the furnace. Single-beam simultaneous designs have two support positions at the end of the beam. The temperature at each support position can be measured independently. The temperature measurements can be combined for DTA or DSC measurements.
Two-beam designs provide for differential temperature and weight measurements. The sample beam provides the sample temperature and weight measurements and the reference beam provides the reference temperature and weight measurements for DTA/DTGA or DSC/DTGA analysis.
Alternate designs for horizontal simultaneous DTA/TGA and DSC/TGA instruments utilize multiple beams, some of which are attached to balance mechanisms and some of which are fixed to rigid mounts outside the furnace. For example, a three beam design uses two rigid mount beams, each with a sample platform, to measure the differential temperature of a sample and a reference. The third beam simultaneously measures the weight change of another sample of the same material. Many different design configurations are possible, including designs with and without temperature measurement at the end of the beam, and with and without balance mechanisms attached to the beams.
The accuracy of weight change measurements in conventional horizontal TGA instruments is reduced by the problem of beam growth. The term "beam growth" refers to the dimensional change of the balance beam due to thermal expansion of the beam as the sample and beam are heated in the TGA furnace. If the dimensional change in the TGA beam moves the sample further away from the pivot point of the horizontal balance, then an apparent increase in sample weight will be recorded. Conversely, any movement of the sample toward the pivot point, typically caused by cooling of the beam, will result in an apparent decrease in sample weight. Normally a correction is applied to the TGA weight change measurement to compensate for beam growth. In conventional horizontal TGA designs the correction is often complex and inaccurate due to the simultaneous thermal expansion of two or more dissimilar materials in the balance beam/support assembly. The accuracy of the correction is further reduced when the sample support materials are not rigidly fixed with respect to each other, and with respect to the balance pivot point.
The accuracy of the weight change measurement in conventional horizontal design TGA instruments is also affected by the accuracy and reproducibility of the sample placement. The sample is usually placed in a removable holder which is then placed on, or suspended from, a sample support at the end of the TGA balance beam. Any variation in the distance between the balance pivot point and the sample will cause an error in the weight measurement. In indirect temperature measurement designs the sample holder is typically suspended by a bail wire from the support. In direct temperature measurement designs the sample holder typically sits on a temperature measurement platform or bead. Any movement in the platform or bead due to mishandling, heat annealing, or thermal expansion will adversely affect the accuracy of the temperature measurement.
Conventional horizontal simultaneous TGA/DTA instruments are constructed with a platinum liner that acts as a sample support platform. This liner is welded to a thermocouple bead. The thermocouple wires are routed through the interior of a hollow balance beam to exit at the cold end of the beam, near the balance pivot. This construction has two principal disadvantages. First, the sample holder is held in position only by the thermocouple wires. This is not a very secure construction, and after several runs to high temperatures the thermocouple wires become soft and malleable, allowing the sample holder to move. Furthermore, the act of loading a sample holder can move the thermocouple wires, and therefore, the sample holder. Any movement of the sample holder adversely affects the sample weight measurement, because it results in a change in the distance between the pivot point of the balance and the sample. Second, the platinum liner itself tends to become soft and to lose its shape after several runs. This also adversely affects the consistency and reproducibility of the temperature measurement.